The number of girls to the number of boys in Primary 2 Goodness was 1 : 4. The number of girls to the number of boys in Primary 2 Kindness was 4 : 5. Both classes had the same number of girls. When 18 boys from Primary 2 Goodness joined Primary 2 Kindness, the ratio of the number of girls to the number of boys in Primary 2 Goodness became 2 : 5. Find the total enrolment of both classes.

Primary 2 Goodness		Primary 2 Kindness
Girls	Boys	Girls	Boys
1x4	4x4	4x1	5x1
1x4		1x4
4 u	16 u	4 u	5 u

The number of girls in both classes is the same.  Make the number of girls the same.  LCM of 1 and 4 is 4.

	Primary 2 Goodness		Primary 2 Kindness
	Girls	Boys	Girls	Boys
Before	4 u	16 u	4 u	5 u
Change		- 18		+ 18
After	2x2 = 4 u	5x2 = 10 u	4 u	5 u + 18

The number of girls in Primary 2 Goodness remains unchanged.  Make the number of girls in Primary 2 the same.  LCM of 4 and 2 is 4.

Number of boys from Primary 2 Goodness that joined Primary 2 Kindness
= 16 u - 10 u
= 6 u

6 u = 18

1 u = 18 ÷ 6 = 3

Total enrolment of both classes
= 4 u + 16 u + 4 u + 5 u
= 29 u
= 29 x 3
= 87

Answer(s): 87